A high-quality generally clear coating having good resistance to lifting, cracking, and abrasion is important to the production of commercially acceptable golf balls. At the same time, heightened environmental awareness has induced governments to enact laws which place tough new restrictions on emissions of volatile organic chemicals which may be present in primer coats. Although many primer coats are waterborne systems today, primers which contain epoxy resins dissolved in organic solvents may still be used to improve adhesion of a top (finish) coat to a golf ball.
The outer surface of a golf ball must accept a coating which will not chip, peel or flake when subjected to very strong impact force and friction force of golfing. Unfortunately, the adhesion of such coatings to specific plastics used in the production of golf balls is weak. Attempts have been made to improve adhesion of such coatings to the surface of the golf ball. Some approaches to improving adhesion have included roughening the surface of the ball by subjecting to a flame or sandblasting. Both methods, however, cause damage to the precisely designed and formed dimples of the most recently developed golf balls. The damages have an adverse effect on ball flight. Alternatively, golf balls have been pre-coated with a treatment solution containing ethyleneimine, carbodiimide or a derivative thereof. U.K. Appln. 2,227,671 suggests applying an outer covering layer to a golf ball by molding a cover member around a core member in which prior to molding the cover material is provided with the outer layer, i.e. a biaxially-oriented film or a co-extruded film laminate. This outer covering layer is intended to improve the adhesion of a paint or varnish to the ball and/or improve the reception of an identification mark or stamp.
However, each of the above methods still necessitates the application of a primer coat between the molded ball and the top coating. The use of a primer, while currently conventional, has several disadvantages in that it generally entails the use of hazardous chemicals, high temperatures, releases volatile organic compounds, and is relatively high in cost. As a result, other methods of improving adhesion are under exploration. One attempt at improving adhesion of a paint or lacquer to a golf ball while avoiding the use of a primer coat entails treating a formed un-coated golf ball with an unpolymerizable gas plasma before applying a finish paint coat. U.S. Pat. No. 4,613,403 discloses subjecting golf balls to an inert gas which is energized by applying a voltage and reducing the pressure to the range of 10.00-0.01 Torr. This is performed by placing the golf balls in an apparatus with a sealed casing to create a vacuum for plasma treatment. Since undesirable strong ashing can occur when oxygen is present during treatment, it must be excluded from the casing. The treatment conditions of temperature, gas content, and pressure must be carefully controlled and contained. Accordingly, there is need for a simpler technique to improve adhesion while avoiding the use of primers.
It is known that the adhesion of certain plastic materials to various other materials may be improved when the plastic material has been irradiated. This has been done with, for example, automobile bumpers and side molding, car mats, adhesive tape, and plastic bottles, but these are not products which are expected to suffer the extreme abrasive forces to which a golf ball is constantly exposed. Moreover, these products are not required to absorb the abrasive forces while having a continuing requirement of no peeling of the top coating.
The applicants have determined that certain exposures of golf balls to UV light can improve adhesion of the plastic surface to a level sufficient to eliminate the need for applying a primer before the application of a top coat. The use of the term "top coat" includes paints, varnishes, polyurethanes, solvent-based urethanes, or any other conventional outer finishes used in the manufacture of golf balls. The method can be practiced without requiring highly specialized equipment to regulate temperature, pressure, and gas concentration. Most importantly, the method results in enhanced adhesion while maintaining a commercially acceptable level of appearance and durability of the golf ball.
Accordingly, it is an object of the invention in the production of golf balls to eliminate any need for a primer before application of the top coat while avoiding the detriments of gas plasma treatments.
It is another object of the invention to improve the adhesion of a top coat to a golf ball with minimal use and release into the atmosphere of hazardous chemicals.
It is another object of the invention to improve the adhesion of a top coat to a golf ball without damaging the surface of the golf ball.
It is still another object of the invention to treat golf balls at ambient temperatures and pressure.